3. Methods for assessing the probability of failure of an MMR

3.1 Allocation of default probability target

As we have already seen, the aim of MMR is to reduce the criticality of hazardous phenomena that may occur at a plant. For an MMR to be able to reduce a risk to an acceptable level, the manufacturer needs to set a target for its probability of failure. An MMR with a probability of failure under load (PFD) of 10 ^–1 may be acceptable if the severity of the hazard that could arise in the event of its failure is limited. On the other hand, if the severity is significant, then it may be necessary to increase the safety target by recommending, for example, a PFD of 10 ^–3 /sollicitation.